Doppler ultrasound is widely used to measure the blood flow velocity in clinical applications. FIG. 1 shows the working principle of Doppler ultrasound for measuring blood flow. To perform a blood pressure measurement, a cuff is provided around the patient's arm and an air bag in the cuff is inflated to increase the pressure in the cuff. When the pressure is increased to a predefined level (higher than the systolic pressure), the artery is blocked and then the air bag is deflated slowly. When the pressure in the air bag is equal to the systolic pressure, the artery is opened and the blood flow is restored. When a Doppler ultrasound 11 is applied to monitor the deflation procedure, it will detect a signal when the artery is open. The corresponding pressure in the cuff at this moment is the systolic pressure. The Doppler ultrasound is used as the golden standard method to measure the systolic pressure.
However, as shown in FIG. 1, the Doppler ultrasound 11 sends a Doppler ultrasound signal 12 towards the blood flow 15 in an artery 14 and the signal is limited to a small scope. If the artery is not in the scope, the Doppler ultrasound cannot measure the reflected signal 13. Therefore, the position of the Doppler ultrasound transducer should be adjusted so as to be placed on the artery, so that it can acquire a good SNR to clearly detect the reflected Doppler ultrasound signal of the blood flow. Further, this position should be maintained during the measurement procedure.
In order to make sure that the Doppler ultrasound transducer is well placed, the conventional procedure to position the transducer needs an operator's judgment, for example, a doctor or physician, according to his/her experience.
Specifically, the operator places the transducer on the brachial or ankle of a patient, such that it can be approximately above the artery. After this step, the operator may listen to the audio output of the Doppler ultrasound, and according to his/her experience, if a clear and regular sound output, which reflects the blood flow of the artery, can be heard, the operator judges that the transducer is well placed. Otherwise, the position of the transducer should be adjusted until a clear and regular sound can be heard.
Since the above procedure to position the transducer needs an operator's judgment, based on the audio output of the Doppler ultrasound, it is not automatic and thus not very convenient. Furthermore, in the ABI measurement, ultrasound signals on four limbs will be measured, thus the audio outputs will interfere with each other during the transducer positioning procedure and it becomes more difficult for the operator to judge whether the transducer is well placed.
In addition to the conventional procedure to position the transducer, it could readily occur to those skilled in the art to use a processing unit to process the Doppler ultrasound signal in order to determine whether the signal is a pulse signal.
However, although the signal may be a pulse signal, it may not necessarily reflect the blood flow of the artery. For example, the pulse signal may be caused by the flapping of the muscle near the transducer.
Therefore, on the one hand, to determine whether the transducer is well placed by simply determining whether the Doppler ultrasound signal is a pulse signal may not be accurate enough and may cause some mistakes.
On the other hand, to make this positioning procedure accurate, when the Doppler ultrasound signal detected is determined to be a pulse signal, the ultrasound pulse signal should be applied using a complicated algorithm and processing to determine that the pulse signal actually is the signal reflecting the blood flow of the artery.